The present invention relates to a method and apparatus for treating white water of a paper machine. The invention especially relates to a fiber recovery process, in other words to the improvement of the "save-all" process.
Fiber recovery processes are used when usable fibrous material is entrained with the effluent from the fiber treatment processes, such as, for example, with white water being discharged in the web formation from the wire section of a paper machine. Usable fibrous material also separates to such filtrates which are separated from the fiber suspension by different slotted or perforated screens or by mainly liquid-permeable wires. It is characteristic of this fibrous material that the majority thereof comprising the finest fiber fraction in the fiber suspension.
The fiber recovery processes are carried out mostly by different filters, such as disc filters comprising a number of discs wire-coated on both sides and mounted on the horizontal axis, the discs being sunk into a vessel containing suspension to be treated. By subjecting the wire surfaces to a pressure difference, i.e. a reduced pressure from the inside or an overpressure from the outside, liquid is removed from the fiber suspension to the inside of the discs and further through the axis to the outside of the apparatus and a fiber matting is formed on the wire surfaces. Also drum filters are used in the fiber recovery processes, the filters being respectively formed by a wire-coated drum mounted to a horizontal axis, and the inner surface of which wire surface is provided with so called filtrate compartments, through which liquid is discharged from the fiber suspension. The problem with the wire surface in both filter types is that it should be able to let enormous amounts of liquid through, but at the same time prevent the fine fibrous fraction from escaping to the filtrate. The best way to carry out the filtration of white water is to first form a thin layer from the "sweetener stock" (long stock), through which layer the actual white water filtration takes place.
Usually sweetener stock is added to the white water so that immediately at the beginning of the filtration a thin filtering layer of sweetener stock is formed on the wire surface. During the first seconds of the filtration most of the fine fibrous material passes with the filtrate, which is usually returned to the inlet of the filter prior to the sweetener stock accumulating and forming a sufficient layer for an effective filtration. If such a layer is not formed at all, all fines tended to be recovered might pass through the wire surface to the filtrate water. To avoid such a situation, the amount of sweetener stock that is fed usually is such that the solids ratio between sweetener stock and white water is about 0.5-1, 1-1.5, or 1.5-2.
The longer the stock of the sweetener stock is, and the less it contains fine fraction, the better it will bind the fines, in other words prevents their entrance in the filtrate water. Thus the best sweetener stock is formed by the long stock, which has a small share of short stock and fine fraction. This impermeability is, however not the only criterium set for the quality of the sweetener stock. It is also significant how thick the fiber netting to be formed is and what kind of thickening properties it has. If the sweetener stock is optimal and the fiber netting thus of the right type, the fine material of the white water is attached thereto throughout the whole layer and not only on the surface of the sweetener stock, so that the fiber netting remains open and thus the flow channels through the fiber netting for the liquid to be filtered remain better and open longer and the capacity of the filtering apparatus is thus higher.
In all previously known save-all processes the tendency has been to affect the operation of the fiber recovery filter merely by the choice of the sweetener stock.
In accordance with a preferred embodiment of the invention, by treating the sweetener stock to be fed to the fiber recovery filter by a fractionation apparatus prior to mixing the sweetener stock to the white water in such a way that the short-fibred fine material is removed from the sweetener stock, the quality of the sweetener stock is thus improved and the efficiency of the operation of the fiber recovery filter increased.
The effect of the fractionation of the sweetener stock comes very apparent when, for example, mechanical pulps, TMP, SGW or PGW, having a high content of the fine material, very often over 30% and in some cases even 50%, are to be used as sweetener stock. The situation is the same also when secondary pulp is used as sweetener stock, which will in the future become more and more usual as the use of the secondary pulp increases.
According to one aspect of the invention, a method practiced comprising the following steps: (a) Fractioning undiluted sweetener stock into fine and coarse fractions. (b) Mixing the sweetener stock coarse fraction, but not the fine fraction, with the white water. (c) Feeding the mixture of sweetener stock and white water to the recovery apparatus. (d) Recovering useable fibrous material from the mixture in the recovery apparatus. And (e) returning the recovered useable fibrous material to the paper machine.
The following advantages are achieved by utilizing the apparatus and practicing the method in accordance with the invention:
the capacity of the filter increases, so that a smaller filter is sufficient;
the filtrate clarity of the fiber recovery filter is improved, because the fine material of the sweetener stock no longer loads the filter;
more secondary pulps may be used as sweetener stock;
long-fibered softwood pulp may be replaced by mechanical pulps or recycled fibers with better yield;
fluctuation, for example, in the fiber recovery grade due to possible process fluctuations, may be controlled. For example, if the quality of the sweetener stock varies, it immediately affects the operation of the fiber recovery filter, but by fractionation of the sweetener stock, the quality thereof may be stabilized thus stabilizing the whole process.
It is also a fact that the solids content of the white water to be fed into the recovery when recovering the fibers varies, for example, according to the retention of the wire section in a paper machine. The maximum amount of solids in the white water when producing paper from pulp components containing a significant amount of fine material may be even 6000-8000 mg/l, i.e. 0.6-0.8%. Naturally, the capacity of the fiber recovery filter and the filtrate clarity depend on the solids content of the white water to be fed to the fiber recovery. The lower the solids content of the white water is, the higher flow the fiber recovery filter must be able to treat and the clearer the filtrates are.
According to another preferred embodiment of the invention the white water entering the fiber recovery is led--prior to mixing with the sweetener stock--through a separation apparatus, separating fibrous material prior to the actual recovery filter into fine and coarse fractions, only the fine fraction of which is mixed with the sweetener stock.
The advantages gained by the invention include:
the capacity of the fiber recovery boiler increases, so that a smaller filter is sufficient;
the filtrate clarity of the fiber recovery filter is improved, in other words the solids content of the filtrates decreases;
the process fluctuations in a paper machine, e.g. the fluctuation in the solids content of the white water due to the fluctuation in the retention, stabilize and do not affect the operation of the fiber recovery filter;
a sort of "middle water" is obtained, of which a portion of the fibers is removed and which may possibly be better used, for example, as a so called "carrier water"; and
the solids ratio between the sweetener stock and the white water may be raised optimal to the fiber recovery without the consistency of the mixture of the white water and the sweetener stock to be fed to the fiber recovery filter rising too high in view of the operation of the recovery (maximum approximately 1.0-1.2%).